1. Field of the Invention
This invention relates to an image forming method and apparatus, such as a copying machine, facsimile machine, printer, and the like, and more particularly to a method using an image forming apparatus having a developing device using two-component developer.
2. Discussion of the Background
In a developer, more particularly a two-component type developer utilized in a developing device of an electrophotographic image forming apparatus, such as printer, facsimile and the like, an amount of charge of the toner and the bulk of the toner tend to decrease while the developer is left for extended periods of time without being used after being manufactured, but before using.
Thus decreased amounts of toner charge and the bulk of the toner causes the problem described below. When decreased toner charge occurs, toner tends to fall from an image and fouls a background portion of the image. The toner falls because an attracting force between the toner and a carrier decreases under a low charging state of the toner. The low charging state occurs because it takes a relatively long time for recovering the decreased charging amount of the toner by agitating the toner and the carrier in a relatively small image forming apparatus having low agitating power, even though such problems do not usually occur in relatively large image forming apparatuses having sufficiently high agitating power.
In addition, when the bulk of the toner decreases, the problem described below occurs in accordance with an erroneous detection of an output value Vt of a toner density detecting sensor. The erroneous detection of the toner density detecting sensor is explained hereinbelow. A mixing ratio of the toner to a carrier, i.e., a toner density of the developer at an early stage, such as at a time of shipment, is set regularly to a proper predetermined value, and the developer has an apparent predetermined bulk since some air gaps are contained in the developer. However, if the developer is left for extended periods of time, the measurable bulk decreases and approaches the true bulk of the developer because the air gaps in the developer tend to collapse with time. As the developer settles, the carrier that is heavier than the toner sinks to the bottom of the developing device. At this moment, the toner density detecting sensor, which is provided at a relatively low part of the developing device and measures toner density per unit area over time, has an amount of the carrier, made of a magnetizable material, pass thereby and induce a voltage value in the sensor. The output of the sensor is sent a control device that appropriately supplies toner based on the output voltage value provided by the sensor.
For example, when the developer bulk decreases, the toner density detecting sensor outputs a signal as if the toner density is low, since the toner density detecting sensor detects the carrier component that has sunk to the bottom of the developing device as a result of the developer having been left for extended periods of time. In other words, even though the overall toner density does not vary, the toner density which represents a ratio of the toner component to the carrier appears to become small, because the toner density sensor only makes a localized measurement at a position where the carrier tends to congregate. This is called an erroneous detection event made by the toner density detecting sensor. According to the erroneous detection of the toner density detecting sensor, the control device tends to compensate for the perceived lack of toner by supplying additional toner into a developing device. Consequently, the ratio of the toner to the carrier increases more than a proper rate.
A toner-containing ability of respective carrier units has a limit, and thus an excess amount of toner that cannot be contained exceeds the containing limit of the carrier, is loosened by electrostatic forces, and frees itself from the carrier of the developer--toner in this condition is so-called floating toner. Floating toner deteriorates image quality and causes toner to fall down from an image and foul the image background.
Thus, the background fouling, solid image scattering and the toner scattering occurs and results in deteriorating the image quality by decreasing of the amount of charge of the toner and decreasing the developer bulk because of prolonged storage of the developer.
Conventionally, a standard developer having a toner density that is predetermined at a time of the manufacture, for example, 2 wt % (weight percent) is used when using the developing device for the first time, replacing deteriorated and used developer. As described above, the developer that is left for extended periods of time should be supplied after recovering the predetermined amount of charge and the bulk thereof by agitating the developer, and further, the required time period for recovering the amount of charge and the bulk of the developer depends on the periods of time in which the developer is left without being used after its manufacture. Furthermore, the required time period for recovering also depends on a charging ability of the developing device. In the conventional developing device, a large amount of the developer, such as, for example, 1 kg has been set and the toner density thereof is low, such as 2 wt %, and the charging ability of the developer of the developing device has been high. Under these conditions, the amount of toner charge for the bulk of the developer is certainly able to recover to approximately the intended predetermined range therefor by performing an agitation operation for a relatively short time period, even though the developer has been left for extended periods of time. Accordingly, the toner density detecting sensor for a standard developer has been considered to output a predetermined toner density value, or the toner density has been controlled by agitating the standard developer until the predetermined toner density value has been outputted with the toner density detecting sensor.
However, in recent years, an amount of developer set in the developing device has become small, for example, about 400 g, so as to accommodate smaller developing devices. However, even though the amount of the developer set in the developing device is made small, the toner itself in the developing device is required to maintain a similar amount of toner as in the conventional developing device so as to adequately perform image forming operations.
Therefore, the trend is for toner density in the developer to get higher, for example, about 4 wt %. In such a developing device, the toner density detecting sensor does not always output the predetermined toner density value with the standard developer when the developer is agitated for a predetermined time period of practical level, for example, about 3 minutes, since the charging ability of the developing device is low. For example, when the standard developer has been left for 3 months, the toner density detecting sensor outputs a predetermined toner density value with the standard developer, however when the standard developer has been left for one year, it takes 10 minutes until the developer outputs the predetermined toner density value. Accordingly, in the developing devices that contain small amounts of the developer that is set therein and uses developer with relatively high toner density, if the predetermined toner density value has been considered to be outputted by agitating the standard developer for predetermined time period as in the conventional case, the toner density value is erroneously detected as a result. Consequently, the aforementioned shortcomings such as the toner falling down from an image and background fouling occur. On the other hand if the developer is agitated enough so that the predetermined toner density value is outputted, the agitating time period has been required to be set more than 10 minutes, which has not been practical. Further, if the standard developer is continuously agitated until the predetermined toner density is outputted, there is a possibility that the agitating time period covers a long time, such as 10 minutes, and this is considered to be an erroneous operation.